The energy-control method and apparatus for digital diversity signaling relates to a transmission method and apparatus for diversity signaling, and more particularly, to an energy-control method and apparatus for noncoherent, digital, diversity signaling over fading channels each with generally different noise and transmission loss.
Digital communications systems frequently must operate in fading channels, meaning that the strength of a signal at the receiver is a random variable. In the most widely used analytic model for fading, the Rayleigh-fading channel, the likelihood of deep fading greatly increases the error probability relative to that for the nonfading case, other things being held constant, where the error probability is the likelihood that a transmitted “0” will be interpreted as a “1” by the receiver, or vice versa. To compensate for fading, diversity signaling is often used. Diversity is the transmission of the same bit or set of bits to the same receiver multiple times, or the same bit or set of bits to multiple receivers. This can be done by various means, including the use of several carrier frequencies, known as frequency diversity, or spatially separated antennas called spatial diversity. Other types of diversity include time, polarization, and path diversity.
Standard methods of combining received diversity signals include maximal ratio combining, equal-weight combining, and selection diversity. Maximal ratio combining maximizes the signal-to-noise ratio (SNR) of the statistics, which a coherent receiver computes when determining the transmitted signal, in exactly the same fashion as a matched filter. Equal-weight combining simply sums the received diversity channels, after suitable prior processing, such as basebanding and filtering for the low-pass waveforms being sought. Selection diversity uses a subset of all diversity channels, namely those with the largest SNRs, which are then combined with equal weights.
Conventional diversity systems typically do not use different energies on different diversity channels. One reason for this is that, in many applications, receivers or transmitters must be lightweight, battery powered, and portable, and may need to operate in conjunction with several similar devices. This limits the measurements, calculations, or adaptations the device or set of devices can practically perform to only the simplest types of diversity combining. Another reason is that system designers often assume for simplicity that the noise and transmission loss are equal for each prospective diversity channel. In general, however, neither noise nor transmission loss is apt to be identical on all possible diversity channels. Therefore, it can be appreciated that a method for optimum energy-control in a diversity signaling system is needed.